Grazer and nutrient impacts on epilimnetic ciliate communities

Abstract

The effects of large grazers (Daphnia) and nutrients (nitrogen and phosphorus) on epilimnetic ciliate communities were investigated in eight large in situ enclosures. Contrasts in Daphnia abundance were created by stocking planktivorous fish (Phoxinus eos) in four of the eight enclosures. Contrasts in nutrients were created by adding nitrate and phosphate to two enclosures with fish and to two enclosures without fish. Both the biomass and mean length of total zooplankton were significantly greater in the fish‐free enclosures than in the enclosures with fish. Addition of nutrients and fish produced a four‐fold variation in concentration of total epilimnetic phosphorus and six‐fold variation in algal biomass (Chl a). Daphnia were virtually absent in the enclosures with fish. The abundance of ciliates was one to three orders of magnitude lower when Daphnia were abundant. Addition of nutrients did not increase ciliate abundance in enclosures with Daphnia, but ciliate mean lengths were larger in these enclosures. Ciliate abundance increased with increasing zooplankton biomass when Daphnia were lacking, but it declined rapidly when Daphnia were abundant, producing a hyperbolic pattern. When the data for all the treatments were considered together, there was no relationship between copepod biomass and ciliate abundance, but positive relationships, although not highly significant, emerged when the data for enclosures with and without fish were considered separately. Our results suggest that large grazers are the most important factor regulating ciliate communities as well as the responses of ciliates to nutrients and resources. Shifts of zooplankton from copepods to Daphnia dominance may result in large reductions in ciliate abundance, regardless of the trophic status of lake ecosystems.